Method of casting in investment molds having a radiation shield

ABSTRACT

In making columnar grained or single-crystal castings, the mold is heated prior to pouring and a meltable metallic shield is placed on the chill plate to serve as a heat shield between the mold and the plate during heating of the mold and also to prevent condensation of impurities on the chill plate.

United States Patent Robert B. Barrow;

Bruce E. Terkelsen, both of Cheshire, Conn.

Apr. 24, 1970 Jan. 1 1, 1972 United Aircraft Corporation East Hartford, Conn.

Inventors Appl. No. Filed Patented Assignee METHOD OF CASTING IN INVESTMENT MOLDS HAVING A RADIATION SHIELD 4 Claims, 3 Drawing Figs.

U.S. Cl 164/60, 164/ l 2] Int. Cl 322d 25/06 Field of Search 164/60,

[56] References Cited UNITED STATES PATENTS 3,385,346 5/1968 Fleck 164/121 2,810,170 lO/l 957 Pearce 249/206 X 2,907,083 6/1959 Shakely 249/206 X Primary Examiner-Robert D. Baldwin Assistant Examiner-John E. Roethel Attorney-Charles A. Warren ABSTRACT: In making columnar grained or single-crystal castings, the mold is heated prior to pouring and a meltable metallic shield is placed on the chill plate to serve as a heat shield between the mold and the plate during heating of the mold and also to prevent condensation of impurities on the chill plate.

o ouom QQQQQ As taught in the VerSnyder patent, U.S. Pat. No. 3,260,505 for columnar grained castings, or in the Piearcey patent, U.S. Pat. No. 3,494,709 for single-crystal castings, such castings are best produced in a shell mold, the open bottom end of which rests on a chill plate. The mold is heated to above the melting temperature of the alloy before being poured and a suitable thermal gradient is established in the mold, after pouring by the effect of the chill plate. The presence of the chill plate during heating of the mold necessitates a longer period of heating, and during this time impurities that may be withinthe furnace or within the mold structure including the ceramics or binder may be vaporized. If some of the vaporized impurities enter the mold cavity and are condensed on the chill plate, this condensed material may, when the alloy is poured result in imperfections or porosity in the-cast part. Any substantial amount of deposited material on the chill plate may also result in a violent reaction with the alloy further producing imperfections or impurities within the casting.

SUMMARY OF THE INVENTION A feature of this invention is a heat shield positioned in the open end of the moldand resting on or adjacent to the'chill plateto prevent loss of heat, from the mold being'heated, to the colder chill plate and to prevent the accumulation of foreign matter which condenses on the chill plate; Another feature is the use of a metalfor the shield that will remain solid until the alloy is poured but that'will be dissolved by the alloy and mix with it. Preferably the metal of the shield is one of the components ofthe alloy.

According to the inventiona metallic heat shield, preferably in nonfiat form and of sheet material is placed on the chill plate within the open end of the mold and resting on the chill plate in only a few locations. The shield substantially covers the area of the open end of the mold and is made of a material that is dissolved by the alloy when it is poured into the mold and that will mix with and be compatible with the alloy. This shield allows for quicker heating of the mold and also prevents collection of impurities on the chillplate.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a vertical sectional view through a mold and chill plate with the shield therein and all located within a mold heating means.

FIG. 2 is a perspective view of one form of shield.

FIG. 3 is a sectional view similar to FIG. 1 ofa modification.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring first to FIG. 1 the mold 2 is positioned on a chill plate 4 and is surrounded by a susceptor 6 by which the mold may be heated and by which the rate of cooling of the mold may be controlled. The susceptor is surrounded by an induction coil 8 by which the susceptor is heated and this latter is preferably in the form of a tapped coil such that'the top portion of the coil may be energized separately from the bottom portion. The susceptor is preferably in the form of a sleeve with adequate space therein to receive the mold. It will be understood that the entire assembly may be positioned within a vacuum furnace for use in providing the vacuum desirable in casting many of these alloys and in getting the mold to the desired pouring temperature.

The mold consists of a growth portion 10 at the bottom, the bottom end of which is open to the chill plate and rests thereon. The growth portion is connected by a restricted passage defining portion 12 which selects and establishes the growth of a single crystal from the growth portion 10 into the root portion 14 directly above the restricted passage. Above the root portion is the blade portion 16 with a platform portion 18 therebetween. At the top of the blade portion 16 is a shroud 20 and above that is the sprue 22 by which the alloy is poured into the mold. When he entire mold is positioned 2; within the susceptor a cap 24; may be positioned thereon as shown.

Resting on the chill. plate andfitting'within thebottom operr end of the growth portion 10, a heat-shieldI26 preferably made of sheet material and: prefcrably'ima nonflat condition as. shown so. that only a few areas of the' hejat ;shi :ld are incontact. with the chill plate thereby; reducing the' heat conductivity.

from the shieldto the chill plateThe nonfia tconditionmaybe provided by forming a plurality of creases 28 in, the shield or alternatively it may be produced'by using a corrugated formof shield. The latter is positioned essentially horizontally and in.

position substantially covers thev area of thechill. plate'that is exposed to the growth portion of the mold and as such serves to prevent the radiation of heat from the-moldd irectlyto the chill plate and also preferably serves to reflect heat' back into. the mold.

The material-of the heatshield is selected so. thatiit'willi not melt during the heating of the mold to the desired temperature but will be dissolved by the alloy when-iris poured into the mold and comes in contact with. the: heatashield; Where the device is used in the casting of someof. the-hi'gIr-temperature alloys used for example inngas turbine'engines the material: used for the shield is preferably'one of thematerials contained in the alloy. For example, in casting the alloy. known as Mar- M-200 which is a nickelfbase high-temperature-alloy. the shield may be either. tungsten or. columbi'um: since the: alloy. contains each of these elements. lthasbeen found. that; the heat shield dissolves readilyin the alloy when it-is poured.

The heat shield serves not only. to minimizc'radiation from I the'mold to thechill plate during heating of themoldbut also serves to prevent the. condensation of gaseous material that could be an impurity for theialloy or could constituteforeign material that would be entrapped within thev alloy during solidification. For example, a material that. is caused to evaporate from the mold or the. susceptor orv any foreignmaterial that may be within the furnace itself may produce this undesired gaseous materialthat could: be depositedv on the. chill plate except for thepresenec ofthis shield. .In a particular casting in which the diameter of the growth portion I0of the mold was 3 inches in diameter ashieldof columbium form ed from sheet stock 0.012 of an inch thick: accomplished the expected results. When the alloy was poured, the shield was dissolved completely. withinthealloy.

The arrangement of FIG. 1 is used for producing singlecrystal castings. When the alloy is poured and the. heat shield is dissolved columnar grains are formed vertically from the chill plate within the growth zone 10. The. restricted passage above the growth zone selects one of the columnar grains to grow into the root portion 14 and hence into the remainder of the mold. The columnar grains and the dendritic growth within the single crystalare caused to solidify, in the: desired form by the effect ofthe chill plate and the maintenance of the proper thermal gradient within the mold during solidification. The actual formation of the single crystal within-the mold is described in the above-identified Piearcey patent,.U.S; Pat. No. 3,494,709.

The arrangement of FIG. 3 is intended to produce columnar. grained cast articles and to thisextcnt the growth portion [0a connects directly without restriction with the root portion 14a the portions being shown as-divided by the .dotted line 30- representing the top of the growth zone. Above the platform portion 18a the remainder. of themold is similar to that described in FIG. 1. When a suitablealloy is poured into this mold, the latter having been heated to a temperature above the melting point of the alloy, columnar grains are formed growing vertically from the chill plateand such columnar grains continue directly through the entiremold. Such growth We claim: heat from the mold during heating of the mold; and l. The process of casting high-temperature alloys in a pouring melted alloy into the mold for forming the casting heated shell mold including, such that the material of the shield is caused to melt and providing a mold having an open lower end, be dissolved into the poured alloy. positioning the mold on a chill plate such that the plate 5 2. The process of claim 1 in which the shield is positioned to closes the open end, shield the chill plate from the'heat of the mold and including placing a thin nonflat metallic shield made from at least one the step of supporting the shield such that the greater portion of the metals of the alloy within the mold adjacent to and of the shield is Out ofcomafl w he C ill plate.

substantially covering the portion of the chill plate within The Process of claim 1 including the additional p of h ld, wi h h greater portion f the Shield Spaced forming a plurality of creases in a sheet of metal to form the from the chill plate, to shield the chill plate from radiant shield into a nonflat configuration Prior P g." in heat from the mold as it is heated, the m l 4. The process of claim 1 including the additional step of peramre of the alloy causing solidification of the alloy by the removal of heat from substantially preventing condensation of vaporized impurithe alloy to the Plateties in the casting cavity by heating the shield by radiant heating the mold to a temperature above the melting tem- 

1. The process of casting high-temperature alloys in a heated shell mold including, providing a mold having an open lower end, positioning the mold on a chill plate such that the plate closes the open end, placing a thin nonflat metallic shield made from at least one of the metals of the alloy within the mold adjacent to and substantially covering the portion of the chill plate within the mold, with the greater portion of the shield spaced from the chill plate, to shield the chill plate from radiant heat from the mold as it is heated, heating the mold to a temperature above the melting temperature of the alloy, substantially preventing condensation of vaporized impurities in the casting cavity by heating the shield by radiant heat from the mold during heating of the mold, and pouring melted alloy into the mold for forming the casting such that the material of the shield is caused to melt and be dissolved into the poured alloy.
 2. The process of claim 1 in which the shield is positioned to shield the chill plate from the heat of the mold and including the step of supporting the shield such that the greater portion of the shield is out of contact with the chill plate.
 3. The process of claim 1 including the additional step of forming a plurality of creases in a sheet of metal to form the shield into a nonflat configuration prior to placing it in the mold.
 4. The process of claim 1 including the additional step of causing solidification of the alloy by the removal of heat from the alloy to the chill plate. 